Principles Of Operation

Anti-Lock Control

Principles of Operation


Anti-Lock Control

The anti-lock control system is controlled by the ABS module which continuously monitors and compares the rotational speed of each wheel. The wheel speed sensors are connected to the ABS module by 2 circuits. One circuit provides voltage for sensor operation and the other circuit provides sensor input to the ABS module. As the wheels spin, the wheel speed sensor tone rings pass through the magnetic field generated by the active wheel speed sensor. This causes the strength of the magnetic field to increase (as a tooth passes the sensor) or decrease (as a gap passes the sensor) and generates a square-wave signal that is sent to the ABS module to indicate individual wheel speed.

When the ABS module detects an impending wheel lock-up, brake fluid pressure to the appropriate brake caliper will be modulated (pulsated). This is accomplished by opening and closing the appropriate solenoid valves inside the Hydraulic Control Unit (HCU) while the hydraulic pump motor is also actuated. Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves to their normal position and the hydraulic pump motor is deactivated.

The ABS module is self-monitoring and will carry out self-tests at pre-determined times. When the ignition switch is turned to the RUN position, the ABS module will carry out a preliminary electrical check of the wheel speed sensors and their circuits by sending voltage through the sensor and checking for the voltage to return. When the vehicle is traveling at speeds above 20 km/h (12 mph), and the brake pedal is not being pressed, the pump motor will be commanded ON for approximately 0.5 second to check pump motor operation. Also, during all phases of operation while the vehicle is in motion, the ABS module checks for correct operation of the wheel speed sensors by comparing wheel speed input to other sensor input. If a malfunction is detected, the ABS module will deactivate the anti-lock control system and will also send a message over the High Speed Controller Area Network (HS-CAN) bus. When the Instrument Cluster (IC) receives this message, it will illuminate the yellow ABS warning indicator and the stability/traction control indicator (sliding-car icon).


Electronic Brake Distribution (EBD)

On initial application of the brake pedal, full pressure is applied to the rear brakes. The ABS module then uses wheel speed input to calculate an estimated rate of deceleration. Once vehicle deceleration exceeds a predetermined threshold, the ABS module closes the appropriate isolation valves in the HCU to hold the rear brake pressure constant while allowing the front brake pressure to build. This creates a balanced braking condition between the front and rear wheels and minimizes the chance of rear wheel lockup during hard braking. As the vehicle decelerates, the valves are opened to increase the rear brake pressure in proportion to the front brake pressure.

If ABS is disabled due to DTCs being present in the ABS module, Electronic Brake Distribution (EBD) will continue to function unless the DTCs are for wheel speed sensors or the HCU. When EBD is disabled, the red brake warning indicator, the ABS warning indicator and the sliding-car icon will be illuminated.


Traction Control System

The traction control system uses the ABS and the PCM to limit wheel spin. When the ABS module detects a wheel spinning excessively, brake pressure to the appropriate brake caliper is modulated. This is accomplished by opening and closing the appropriate solenoid valves inside the HCU while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN bus that a traction control event is taking place. When the PCM receives this message, it will assist with traction control by adjusting engine timing and decreasing fuel injector pulses, when the IC receives this message, it will flash the sliding-car icon (twice per second). Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves in the HCU to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN bus indicating that the traction event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC extinguishes the stability/traction control indicator.

Once vehicle speed reaches or exceeds 100 km/h (62 mph), traction control is accomplished only through the PCM.

If the anti-lock control system is disabled due to DTCs being present in the ABS module, the traction control system will also be disabled. When the traction control system is disabled due to DTCs being present, both the ABS warning indicator and the sliding-car icon will be illuminated.


AdvanceTrac(R) System

The AdvanceTrac(R) system is controlled by the ABS module and uses the same wheel speed sensors and tone rings that are used for the anti-lock control system. The AdvanceTrac(R) system also uses input from the steering wheel rotation sensor on Hydraulic Power Assist Steering (HPAS)-equipped vehicles, the Power Steering Control Module (PSCM) on Electronic Power Assist Steering (EPAS)-equipped vehicles, and inertial signals from the Restraints Control Module (RCM) (yaw rate and lateral acceleration) to help maintain vehicle stability. The ABS module uses all of these inputs to continuously monitor the vehicle's direction of travel relative to the driver's intended course. If the ABS module determines from all these inputs that the vehicle is unable to travel in the intended direction, it modulates brake pressure to the appropriate brake caliper(s) by opening and closing the appropriate solenoid valves inside the HCU while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN bus that a stability event is taking place. When the PCM receives this message, it will assist with vehicle stability by adjusting engine timing and decreasing fuel injector pulses. When the IC receives this message, it will flash the sliding-car icon (twice per second). Once the stability condition has been corrected, the ABS module returns the solenoid valves in the HCU to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN bus indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC extinguishes the sliding-car icon.

If the anti-lock control system is disabled due to DTCs being present in the ABS module, the AdvanceTrac(R) system will also be disabled. When the AdvanceTrac(R) system is disabled due to DTCs being present, both the ABS warning indicator and the sliding-car icon will be illuminated.


Traction Control Switch

Unlike the standard anti-lock control system, the traction control system can be deactivated through the traction control switch. On vehicles equipped with a navigation system, the switch is part of the Front Controls Interface Module (FCIM) module but is hard-wired to the IC. When the switch is pressed, a signal is sent to the IC indicating a request to deactivate the traction control system.

On vehicles not equipped with a navigation system, the switch is part of the HVAC module, but still has a separate hard-wired circuit connected to the IC. When the switch is pressed, a signal is sent to the IC indicating a request to deactivate the traction control system.

When the IC receives the disable signal, a message is sent over the HS-CAN bus indicating a request has been made to deactivate the system and illuminates the traction control OFF indicator (sliding-car OFF icon). After receiving this message, the ABS module and PCM will ignore any traction events until the switch is pressed again or the ignition key is cycled.

The AdvanceTrac(R) system cannot be disabled when a MyKey(TM)-restricted key is in use that has the AdvanceTrac(R) always-on feature configured to "on". If an attempt is made to disable the AdvanceTrac(R) system while a MyKey(TM)-restricted key is in use, ADVTRAC ON MY KEY SETTING will display in the IC message center. For information about MyKey(TM), refer to Antitheft and Alarm Systems.

Stability/Traction Control Indicator (Sliding-Car Icon) and Stability/Traction Control OFF Indicator (Sliding-Car OFF Icon)

Status of the traction control and AdvanceTrac(R) systems is indicated by a stability/traction control indicator (sliding-car icon) and a stability/traction control OFF indicator (sliding-car OFF icon) located in the IC (or message center, if equipped). When the driver disables the systems through the use of the stability/traction control switch, the sliding-car OFF icon will illuminate solidly. When one or more of the systems is currently active, the sliding-car icon will flash, and when the system is disabled due to DTCs being present in the ABS module, the sliding-car icon will illuminate in conjunction with the yellow ABS warning indicator.


Stability Control Sensors

The RCM inertial signals consist of the yaw rate sensor and lateral accelerometer. The RCM is usually mounted as close as possible to the center point of the vehicle to get the most accurate readings. Information from the RCM sensors is sent to the ABS module over a dedicated Controller Area Network (CAN) bus.

- The yaw rate sensor measures the relative vehicle motion about the vertical axis through its center of gravity.
- The lateral accelerometer measures the acceleration that corresponds to the force involved when the vehicle moves sideways.

Lateral acceleration has 2 forms. The first is the centrifugal acceleration that is generated when the vehicle travels around in a circle. The second is the acceleration due to gravity. On level ground there is no contribution from this acceleration. However, if the vehicle is parked sideways on a bank or incline, the sensor measures some lateral acceleration due to gravity, even though the vehicle is not moving.


Steering Wheel Rotation Sensor

The steering wheel rotation sensor (on vehicles equipped with HPAS) measures the rate of rotation of the steering wheel by monitoring a slotted-wheel as it passes through the sensor gap. The rotation sensor uses 2 signal lines to transmit information to the ABS module about whether the steering wheel is being turned left or right and how far it is being turned. The rotation sensor does not indicate the position of the steering wheel relative to straight-ahead.

On vehicles equipped with EPAS, the steering angle and rotation information is transmitted to the ABS module over the HS-CAN bus from the Power Steering Control Module (PSCM).

Steering Angle Sensor Module (SASM)

Vehicles equipped with EPAS are also equipped with a Steering Angle Sensor Module (SASM). For vehicles without Active Park Assist, the SASM has no internal circuitry and does not contribute in any way to AdvanceTrac(R) functionality. In this case, the SASM is used to provide structural support for the clockspring and for canceling the turn signals. However, on vehicles that are equipped with Active Park Assist, the SASM is fully functional. The ABS module receives steering wheel angle information that is transmitted on the HS-CAN bus from both the PSCM steering gear and the SASM.

The SASM measures the rate of rotation of the steering wheel by monitoring the steering wheel rotation ring as it passes through the sensor gap. The SASM uses the HS-CAN bus to transmit information to the ABS module about whether the steering wheel is being turned left or right and how far it is being turned.

ABS Module Configuration

The ABS module must be configured when any one of the following components are installed new:

- ABS module
- HCU

To configure the ABS module, refer to Programmable Module Installation (PMI) in Information Bus. Programmable Module Installation (PMI)